They are very absorbent, so cleaning mushrooms under running water will fill the mushrooms with liquid, giving a watery finish to your salad dressing and the bottom of your salad bowl.

The best way to clean mushrooms is to instead brush them off with a damp kitchen towel. Or if you are going to cook the water off anyway (like in a sauté), you can wash them under water. A fun mushroom fact is a cremini (now being called a Baby Bella), turns into a Portobello 24 hours later.